Low-level sensors crucial to safe shuttle ascent

On the actual flight in July 1999, the velocity shortfall was only about 12 feet/second (about 8 mph out of the required 18,000 mph for stable orbital flight), which translated into being perhaps eight miles low. The Chandra telescope's booster rocket had plenty of extra power to make up for this, and the mission proceeded normally.

The underspeed was hardly noticed, because another far more serious failure on the launch had brought the crew much closer to loss of vehicle and crew. Two separate short circuits in wires connecting main engine control computers threatened to shut off one — or more — of the engines early in the flight, throwing the mission into an abort mode without any airfield in range. The crew would have had to bail out in the final moments of their descent toward the ocean.

More than once — but when?
A different flavor of low-level sensor failure might also be able to send an ascending shuttle into a similar emergency descent. If two of the sensors failed “dry” even while there was propellant remaining, the engine control computers could be misled into declaring a nonexistent “low level” situation.

To protect against this, shuttle computers ignore the low-level sensors until the clock is within 10 seconds of the expected normal shutdown. But if two sensors have broken in the same way, and alert operators in Mission Control in Houston haven't noticed this in time to disarm them, the main engines could then shut down erroneously, 10 seconds and about 700 mph short of a safe orbit. An emergency return to Earth — perhaps over the ocean, or just perhaps within range of an emergency landing field — would then be inevitable.

This 1999 experience and these dramatic possibilities underscore the criticality of launching only with fully functioning low-level sensors. And apparently Collins' most recent flight wasn't the only time this has happened.

In response to a query from MSNBC.com, NASA Johnson Space Center spokesman Pat Ryan e-mailed that experts told him it had happened once before. "There have been two low level cutoffs in the history of the program, both of them for real, out of gas situations," he e-mailed. "In other words [this was] when the sensors operated properly."

"I don't believe there has ever been an in flight failure of [low-level] sensor at all," he added, and other sources have confirmed this.

However, as of this writing, officials and private sources have not been able to identify the other case of an early engine shutdown commanded by the sensors. "I know we have had a [low-level] cut off on at least a couple of flights, including one of mine," an astronaut e-mailed MSNBC.com while requesting that his or her name not be used. "One of my flights, I think, had [engine cutoff] due to low fuel." But Internet-accessible files have provided no confirmation of this.

So although the documentation difficulties have frustrated press inquiries into the full history of this failure mode, the big picture is clear.

Collins and her crew, and everybody else watching this flight, can be relieved that they didn't wind up making a new entry into the small — but non-zero — history of low-level sensor mission anomalies.

MORE FROM HUMAN SPACEFLIGHT

Human Spaceflight Section Front

Spacewalkers spruce up the station Phew! Spacewalkers remove explosive bolt NASA to fix damaged launch pad Space shuttle makes ‘beautiful landing’ NASA eases concerns over shuttle objects Why NASA watches out for true UFOs FirstPerson: Your blasts from the past Space Coast worries about shuttle shutdown Get a look at NASA’s next spacesuit Astronauts talk sports during time off Human Spaceflight Section Front   Add Human Spaceflight headlines to your news reader: